What are the guidelines for regrinding considerations for HP carbide coated drills? (This question assumes that depth of hole or flute length are not a consideration.) As with most things, there are some trade-offs to consider as follows:

Fortunately, most carbide HP drills have a parallel web (without increasing web taper) that eliminates the need to be thinned, as is the case with the increasing thickness web styles

Each time the drill is recoated, the coating builds up in the flute area and on the outer margins of the drill. This buildup can cause a 10% to 15% loss in tool life as the coating gets thicker and more likely to flake off. We have some customers that have us re-sharpen only because of this issue and feel it is not worth the extra expense and lead time to recoat as well.

However, the loss in tool life can be even higher at up to 25% or 30% if you do not re-coat, so we generally recommend recoating.

HP drills have .001 per inch or more back taper on the diameter thus the drill diameter gets smaller each time is re-sharpened. This is offset somewhat by the layering effect of each recoat as stated above.

In order to maintain good performance it is very important for HP drills be reground to the original manufactures specifications.

It is not uncommon for us to re-sharpen drills 15 to 20 times and they still perform well.

When asked to establish the drilled hole size for an STI (Standard Thread Insert) Helical Coil Thread, there is actually a very simple pair of formulas.

Maximum Drill size = Basic Major Diameter + (.924 x Pitch)

Minimum Drill size = Basic Major Diameter + (.799 x Pitch)

It should be noted that the basic major diameter required for these formula is the decimal equivalent to the thread size inside the insert. For example the basic major diameter of a ¼-20 STI threaded insert is equal to .2500. The formula calculates the drilled hole diameter for the oversize thread, which is the outside thread diameter of the insert.

Over the years, the use of carbide drills, end mills and other round tools has increased at an exponential rate. Not only do they provide longer tool life, they allow for increased speeds, making them more productive. To operate at these elevated speeds, the tools must be built with greater precision to minimize vibration. Combined with properly maintained balanced tool holders, vibration at high speeds has been virtually eliminated, producing parts with superior finish and greater accuracy.

The latest generation of carbide taps break from tradition and are being built with the same design philosophy as other carbide round tools, including the use of cylindrical shanks to utilize precision tool holders for high-speed tapping. Combining high accuracy h6 tolerance drill shanks with tools having a total run out of less than 10 microns, allow the tap to produce very accurate and consistent threads with greatly improved thread finish. By comparison, traditional HSS taps can have almost 10 times as much total run out!

In addition to contributing to greater precision, one of the other major benefits to the new cylindrical tap shank approach is that they are designed to be the same as the carbide tap drill for a given thread size, allowing the use of existing drill holders to drive the taps. This eliminates the expense for separate tap holders. Since high performance carbide taps are designed to be used in rigid or synchronous CNC machines with precise feed capability, they can be held rigidly in hydraulic, shrink fit, TG (or TGHP), or precision ER collets, or adapters. Used properly, the holding power provided by these systems is more than adequate to handle the tapping loads encountered, eliminating the need for the antiquated tap square for driving the tap.

Truncated type setting plug gages are used for setting and evaluating thread ring gages. They are also used at times for setting roll thread snap gages and indicating type thread gages. Truncated type set plugs are never used as working plug gages to check parts directly.

The truncated set plug has two sections -- a "truncated" section and "full form" section. Threads on the front half of the thread length are truncated (flattened - reduced on major diameter). Threads at the back have a full height of thread.

The pitch diameter is uniform over the entire thread length. The combination of the two sections permits checking for flank wear, taper and bell mouth conditions in a thread ring gage as well as assurance of adequate root clearance.

Setting plugs are made in a taperlock style up to size 1-1/2" and trilock style over 1-1/2" up to 8".

The thread length on "truncated type" setting plugs should be at least twice as long as the corresponding ring gage thickness. The ring gage can then be checked for proper fit on each section of the setting plug independently.

Since setting plugs are "masters" for setting thread ring gages they are made to within class "W" tolerance on lead and angles. They are made to the broader class "X" tolerance on Pitch Diameter.

The setting plug establishes the size of a thread ring gage . Setting plugs are also known as master plugs or check plugs. They should be the "truncated type" having both a full form and a truncated section on them. All standard setting plug gages are made to a Class "X" on pitch diameter and a class "W" on lead, and thread angle.

Before using the setting plug it is necessary to know that it is within the proper tolerance and free from nicks, jams or surface defects.

Setting plugs must be reasonably straight on pitch diameter as they tend to wear more rapidly at the front in normal usage. However, using the following procedure helps to prolong the straightness of the setting plug.

Thoroughly clean both the ring and the setting plug. Then visually inspect to make sure all foreign material in the thread has been removed and no nicks or burrs are present.

Lubricate the setting plug with a thin film of oil.

Turn the locking screw counter-clockwise until it is loosened and turn the adjusting screw clockwise, which opens the ring to a larger pitch diameter than the setting plug.

Thread the ring gage onto the setting plug all the way to the back (full form section) so that approximately one thread extends beyond the last thread of the setting plug. (This promotes more uniform wear over the entire thread length of the plug.)

Turn the adjusting screw counter-clockwise until there is a slight drag between the ring and setting plug and then turn the locking screw clockwise until tight. There should now be a noticeable drag between ring and setting plug. Operations 6 may have to be repeated to obtain the proper drag fit.

Turn the ring gage from the full form section onto the truncated section of the front. The drag should be approximately the same to both sections,and now remove the setting plug from the ring.

Now turn ring onto the setting plug 1-1/2 to 2 threads at the front. There should be some resistance or drag even at this short engagement. To test for taper or bellmouth, place the ring on its face on the workbench and test for shake or looseness with the setting plug

Remembering the feel at the 1-1/2 to 2 thread engagements, turn the ring further onto the truncated section. The drag should remain approximately the same

Remove the ring from the setting plug and repeat operations 8 and 9 on the other side of the ring gage. The fit should be approximately the same on both sides of the ring to insure proper straightness.

The minor diameter of a thread ring gage is normally checked with GO and NOT GO plain plug gages. These should be made to the minimum and maximum limits of the minor diameter of the ring gage, and to class X or XX cylindrical gage tolerance. The GO should enter through the ring and the NOT GO should not enter to assure that the minor diameter is correct after the ring is set to the proper pitch diameter.

It is standard practice to cover the adjusting screw and locking screw holes with sealing wax. This helps to prevent unauthorized tampering with the setting of the ring gage. Green wax is used on the GO gage and red wax on the NOT GO rings. This is not mandatory, but provides an additional means of quick identification.

Tightness of Fit

There are no established torque values for the degree of drag. Some judgment and common sense must be used. The resistance or drag for a small size gage should be less than for a large size gage. A spin fit is obviously much too loose. Too tight a fit could damage or cause excessive wear on the ring or the setting plug. On properly made gages with accurate lapped threads it takes very little change in size to effect a noticeable difference in drag. Two different setting plugs both within class W tolerance may feel entirely different in the same ring gage. One could be too tight and the other too loose.

In addition to pitch diameter variations, there may be slight different in the degree of drag at full engagement versus partial engagements. One should not expect absolute perfection. Within reason, these differences are not serious as both the ring and the setting plug may well be within their own tolerances.

Resetting of New Gages

It should be realized that a ring gage set to one setting plug does not necessarily fit another setting plug due to allowable tolerances. When ring gages and setting plugs are ordered together they should be matched properly at the factory before shipping. Sometimes the customer's setting plug is loaned to the manufacturer who matches them. In other cases it may be necessary for the user to reset the ring gage to his own setting plug -- following the recommended procedure outlined above.

When a drawing shows a particular thread size and class of fit, and contains a notation regarding a plating deposit, it is assumed that the standard class of fit limits apply to the threads after plating.

Correct calculation of the effect of a coating or plating on a screw thread is very important, and coordination is required in the manufacturing, plating, and gaging operations. The first important point to understand is that on actual screw threads having a 60° thread angle, the thickness of plate uniformly deposited changes the pitch diameter by 4 times the actual thickness, measured perpendicular to a thread flank. The pitch diameter change is two times the thickness of plate on one side of the thread, which results in 4 times on diameter.

Thus, for every .0001 thickness of plate evenly deposited, the pitch diameter of an internal thread will decrease .0004. On external threads the effect of .0001 of plate increases the pitch diameter by .0004.

For internal threads it is very common to use an "oversize" tap, or one having a larger than normal H limit. The threads produced are large enough to accommodate the plate deposit and will then conform to the class of fit, or pitch diameter limits On external threads the tooling (which is normally adjustable) must produce a pitch diameter smaller than normal, so that after plating the thread size will come up to the required normal "after plate" pitch diameter limits.

There are several variations in the methods used to arrive at appropriate "before plate" limits. These are as follows:

Maximum and Minimum Thickness Specified

If the drawing or specifications give both a maximum and a minimum plate thickness, the following method is recommended.

Internal Threads: The "before plate" pitch diameter limits are larger than the normal "after plate" pitch diameter by the following amounts:

Minimum limit larger by four times the maximum plate thickness.

Maximum limit larger by four times the minimum plate thickness.

External Threads: The "before plate" pitch diameter limits are smaller than the normal "after plate" pitch diameter by the following amounts:

Note that if the "before plate" limits are established as illustrated, the product threads will always fall within the desired "after plate" class 3A and 3B limits, as long as the plate deposit is controlled within the .0004 maximum and .0002 minimum thickness specified. For example, in the extreme conditions if the internal thread is at the maximum size limit before plate, and the plate deposit is only the minimum amount, the size will come down to the maximum "after plate" limit, and will not be oversize. Conversely, if the internal thread is at the minimum size limit before plate, and the plate deposit is at the maximum amount, the size will come down to the minimum "after plate" limit, and will not be undersize. This points out the importance of gaging threaded parts, (both before plate and after plate) for successful results.

Notice also, that the tolerance between the "before plate" limits is less than the tolerance on the parts "after plate". In other words, the plating tolerance steals some of the manufacturing tolerance. The greater the plating tolerance, the smaller the manufacturing tolerance. This should always be into consideration, as there can be cases where "before plate" limits are too close for realistic manufacturing tolerances, and some modifications may be necessary.

There are six main steps to consider as you prepare to order a tap for any application.

1. Determine just what kind of tool you need to satisfy the requirements for your application.

a. Are you just trying to put in a few threaded holes in a piece of relatively easy to machine material? If so, the general-purpose tools that are found in most catalogs are most likely adequate for your application.

b. Maybe you have many thousands of threaded holes to complete on very expensive parts that cannot have any rough or poor quality threads. In this case consider tweaking the application by selecting High Performance Taps that are material specific, and made to give long life as well as a high quality finish. The costs will be higher for each tool, but the results obtained will in most cases give a lower cost per hole, and superior results.

b. Spiral Point Tap: Straight flutes with an angular left hand grind near the front of the tap that shoot the chips forward in front of the cutting action in thru holes.

c. Spiral Flute Tap: A tap having a right hand spiral in its flutes and tend to lift the chips out of the predrilled hole. These work best in blind holes.

d. Form Tap: A tap made to extrude the thread into the predrilled hole without making any chips in the hole. These provide a very fine finish in the hole and work particularly well in softer ductile materials as well as deep hole application.

3. Determine the chamfer required on the tap style you have chosen.

a. Taper: This is a chamfer length of 7-10 incomplete threads on the front end of the tap. This chamfer is used in thru hole applications where the chamfer length is not objectionable.

b. Plug: This is a chamfer length of 3-5 incomplete threads on the front end of the tap and is the most commonly selected of all chamfer lengths.

c. Semi-Bottoming: This chamfer length is 3-4 incomplete threads of the front end of the taps and is used where it is necessary to thread closer to the bottom of a blind hole, but still have a lot of cutting action.

d. Bottoming: This chamfer is the least desirable and is 1-2 threads in length of incomplete threads on the front end of the tap. This is used only were it is important to get very close to the bottom of the predrilled hole with full thread. This chamfer should be avoided where possible, as it requires a very large chip load per tooth and can give much shorter wear life than the longer chamfers.

4. Determine Class of Fit required by the part manufacturer on the print.

a. There are three Class of fit for all internal unified series threads in the USA:

• Class 1B: Loose Fit

• Class 2B: Medium Fit

• Class 3B: Tight Fit

b. There are three class of fit for all external unified series threads in the USA

• Class 1A: Loose Fit

• Class 2A: Medium Fit

• Class 3A: Tight Fit

5. Determine H-limit required on the tap (Tap Tolerance)

a. H-for high above basic Pitch Diameter

b. L-for low below basic Pitch Diameter

c. H- Limits are in .0005 increments

d. Place the H -Limit at approximately 40% of the total tolerance of the required Class of Fit

6. Determine the Surface Treatment, or Thin Film Coating

a. Surface Treatments

• Black Oxide: Works best in ferrous (iron based) materials

• Nitride: Use in non-ferrous materials such as aluminum

• Oxide over Nitride: Works well in steels, stainless, and nickel alloys

b. Thin Films

• TiN: Works in broad range of materials such as steels, irons, and plastics

PVD coating General Purpose solid carbide drills for improved performance and drill life is a common practice today. Some PVD coatings have a higher coefficient of friction when compared to the drill in the bright condition. This issue is especially noticeable when the drills are coated with TiAlN.

This higher coefficient of friction increases the possibility of chip jamming or clogging the flutes, causing drill chipping and breakage. This type of failure will often occur on the first hole. If the drill survives the first hole, the flutes become self polished, due to the rubbing of chips as they pass through the flutes. The smoother surface often eliminates further clogging of the chips.

To avoid the potential for failure, this problem can be overcome by having the coating vender polish the drill flutes after coating. This added polish operation, often referred to as "post polish", smooths the surface and provides a coefficient of friction that is even lower the than that of an uncoated drill, resulting in excellent chip flow.

Most High Performance carbide drills are routinely supplied with post polished flutes to avoid this problem.